Two-stroke experts, I have a tricky problem...

[B0000M]

to set the story, my bike recently started running stupidly rich and using trans oil - simple. thats simple, obviously the gearbox side crank seal right?

replaced crank seal. and it is important to note that during this occurance i DID NOT TOUCH jetting or anything else, i simply rode it, noticed the problem, replaced the seal

right, so the smoke significantly reduced. and the gearbox is no longer using any oil. this test was taken over an hour or so of play riding, then about 2 hours of high revving racing (expert class rider on flat tracks) - so a fairly good test for oil use.

bike runs sweet in mid to high rev range, but down low it is VERY rich (looks like a bike with the choke on)

Filter is clean, also replaced with another known good one in case something funny was going on, including a fresh oil from different can.

the fuel bowl is clean, the main and pilot are not blocked, the air screw path is not blocked. the bike can be made to stall at idle with the air screw, so is doing something...

i havnt yet had the choke section apart, though i do know giving choke makes it run even richer like a choke should.

reeds are fine.

fuel is fresh 98, bought on 2 seperate occasions with a month between


i've run out of ideas aside from perhaps the choke not disabling enough somehow?



anyone else got any suggestions?

[Rupe]

I wonder if the oil has done something to the pipe, muffler, or packing? Just guessing really

[noobi]

Reeds? worn or frayed so cant hold tension causing a rich engine at low revs, works fine in the top end because the reeds are wide open anyway?

[cambocambo]

I had this once turned out that I was running a 4 stroke oil in a two stroke doh

[theblacksmith]

Id go with your gut instinct Sam-check to see if the choke is returning properly.
So- after it idles rough and you give it throttle-does it clear out and start running smoother without load?

[Crasherfromwayback]

I'm sure you have already...but at the risk of sounding stoopid/insulting, change the plug too just in case.

[B0000M]

have done the plug, and have re packed the exhaust during the problem.

have now had the choke apart, looks good but may have fixed it. cant give it a test until the morning though so i dont wake my boy up

[barty5]

Its a bloody 2T what you expect (sorry some had to say what others where thinking) Serious note dosnt really look like you have missed much although saying that Rupe and I had an CR recently started and ran well till it got high in the revs turned out bottom have of the piston skirt was missing/ sitting in bottom of casings.

[Reckless]

Check there's no dried mud blocking them or crimps in your vent hoses B0000M!

Sounds similar to something that was wrong with the first Ktm I bought?? Learnt many years ago when my waiste bottle in my Kart was to tightly sealed??

Just a suggestion??

[B0000M]

well ive had the choke apart, and all looks good, and clean, and with the compressor i can gently blow air through and everything seems to work as it should.

however when the bike is running i have discovered turning the choke on and off makes no difference, despite being able to hear with the compressor with it apart the choke opening and closing the air tract to the back end of the carb and the fuel line into the bowl.

next thing i tried was blocking the fuel intake for the choke in the bowl to see if this would help, which it did. but im not sure if its right because i've lost my sence of what is right now lol.

of course this isnt a permanent fix, merely a test.

cheese, send me your carb to try, i know you wont be using it!

[F5 Dave]

to set the story, my bike recently started running stupidly rich. . .

So it was running perfectly before? So you think it is running rich why? (not trying to be condescending) Just a dark plug or some spooge out the tail pipe? Or that it feels blubbery & lethargic at low throttle applications.

. . .
bike runs sweet in mid to high rev range, but down low it is VERY rich (looks like a bike with the choke on). . .

What do you mean 'looks like'?

Think of any Jetting issue by throttle position, not revs. Thats how slide carbs work so always examine your thoughts. Stupidly rich can still clear out at mega revs, but more importantly you are likely to be on different jets. Main jet for full & pilot etc for just off closed throttle.

Most MX bikes are jetted poorly from new to be safe. A leaking float jet, slightly high level or blocked pilot air passage will make off closed rich. A worn needle jet (even slightly oval, use a strong torch down the carb body) will make things bad here too.
Sadly probably not replacable if you have a Keihin.

Have you tried just giving a wind out of the idle mixture screw 1/2 a turn to see if its better?

[B0000M]

So it was running perfectly before? So you think it is running rich why? (not trying to be condescending) Just a dark plug or some spooge out the tail pipe? Or that it feels blubbery & lethargic at low throttle applications.

until the crank seal went- which went gradually, it ran perfectly. nothing on the carb has been changed since perfect running. its now very blubbery, and lethargic at low throttle positons, barely wants to idle, shitloads of blue smoke. after being at low revs / throttle position, bike takes a few good revs to clear and actually rev up properly



What do you mean 'looks like'?

as above, shitloads of smoke

Think of any Jetting issue by throttle position, not revs. Thats how slide carbs work so always examine your thoughts. Stupidly rich can still clear out at mega revs, but more importantly you are likely to be on different jets. Main jet for full & pilot etc for just off closed throttle.

Most MX bikes are jetted poorly from new to be safe. A leaking float jet, slightly high level or blocked pilot air passage will make off closed rich. A worn needle jet (even slightly oval, use a strong torch down the carb body) will make things bad here too.
Sadly probably not replacable if you have a Keihin.

my bike is jetted perfectly to the conditions, it is the same setup i have been running on this bike with no other mods for 100+ hours. prior to this i spent quite some time getting it right.

Have you tried just giving a wind out of the idle mixture screw 1/2 a turn to see if its better?

tried this, makes no noticable difference no matter how far i wind it out - even if its out altogether!, although if i wind it right in it will get even richer and stall

float level / leaks is something i've not yet checked out, though i'd expect it to overflow if it were set too high, as this bike likes to overflow quite easily with a bit of a jiggle around or a bit of a lean. ill go look into the float now.

air ducts for air screw are flowing nicely

main flows nicely
pilot is clear
choke ducts are clear, and as i said earlier although with air you can hear the difference, with the bike running you cannot.

im keen to hear any suggestions, as im out of ideas.

..........

[F5 Dave]

Well you've done most of the things I'd suggest like change the airfilter & oil. Its always difficult when another factor has crept in at the same time like the crank seal. You say you've replaced the muffler packing as that would have got contaminated, but after the seal was fixed?

Yes I'd expect the carb to overflow, but a borderline issue might not & a high level will cause it to run rich even if it isn't quite flooding. Setting it at the low end of std will answer that if even as an experiment.

If I was evil I could suggest that perhaps the seal was sucking air from the top of it quite some time before it started opening up enough to suck oil & when the seal was replaced the mixture was rich. But that might make your head explode so pretend I didn't suggest that.

[CRF119]

So ive read whats gone on so far and again another long shot but the gearbox oil hasn't gummed up the power valve on the exhust?(Assuming it has one) If it hasn't been cleaned in a while worth a clean anyway.

[cheese]

Just to eliminate it, can you do a compression test?